It is previously known that the damping in a parallel resonant circuit comprising a coil and a capacitor, a so-called LC oscillating circuit, is influenced by whether the coil is close to an electrically conductive object, or whether two resonant circuits are located close to each other. From German Offenlegungsschrift DE 3318900 A1, a simple electric circuit with an LC circuit is known. A resonant oscillation process in the LC circuit is started with a dc pulse, and it is shown how the oscillation process is damped to a greater extent when an electrically conductive object has been brought close to the coil of the LC circuit.
German Offenlegungsschrift DE 3213602 A1 describes how an electronic apparatus may be designed to derive advantage from the above-mentioned fact. After a resonant oscillation process has been started in the resonant circuit with the aid of a pulse generator, after transformation of the oscillation process up to the point where the oscillation amplitude is lower than a selected threshold value, the apparatus gives rise to square pulses to a counter. The number of square pulses for the respective oscillation process corresponds to the number of half-cycle oscillations with greater amplitude than the threshold value, and this number of square pulses is dependent on whether the resonant circuit has been damped or not through an external influence, for example by a metal object being brought against the coil of the resonant circuit.
It is also previously known to use this principle of sensing for non-contacting sensing of the rotational state in a flow meter for liquids, which has an impeller which rotates with the liquid flow. German patent specification DE 3923398 C1 is based on such a known flow meter, which has a rotor with a section of a surface divided around a rotational turn into surface parts with different electromagnetic properties, that is, with electrically conductive and electrically nonconductive properties, respectively. A number of electric coils included in a corresponding number of LC circuits are arranged adjacent to the rotational path of these rotor surface parts. In these oscillating circuits resonant oscillation processes are started, in a periodic sequence, with the aid of dc pulses emitted to the circuits in a suitable chronological order, whereby the respective resonant oscillation processes are damped to differing degrees, that is, at different speeds, depending on whether the coil of the circuit is currently located adjacent to a rotor surface part with electrically conductive or a rotor surface part with electrically non-conductive properties. The oscillation processes are transformed into different numbers of output square pulses for the LC circuits which are damped to a maximum extent and to a minimum extent, respectively. The square pulses are passed to a counter connected to an evaluation unit, which, from these state signals, calculates the desired data regarding rotation and rotational direction.
According to DE 3923398 C1, it is a common embodiment in these known devices that the conductive and the nonconductive rotary surface parts of the rotor each comprise 180° and that four electric coils are uniformly distributed adjacent to the rotational path of the conductive and the non-conductive surface part of the rotor. With four coils at a mutual distance of 90°, at least one resonant circuit will always be damped to a maximum extent and at least one resonant circuit will always be damped to a minimum extent. From the same specification it is known, as a fundamental condition, that for sensing of the rotational direction, that is, in addition to the sensing of the magnitude of the rotation, a third coil is needed, preferably displaced 90°, in addition to two sensing coils which are displaced 180° in relation to each other. Using a fourth coil, placed at an angle of 90° between the four coils, was considered an advantageous embodiment. In that case, the rotational direction may be sensed also in the event of loss of a coil, for example due to some malfunction.
Thus, the known devices are based on the fact that, depending on whether damping through external influence occurs or not, the oscillation process carries out different numbers of oscillations up to the point where the oscillation amplitude falls below a specified threshold value. However, this number of individual oscillations changes as a result of influence from, for example, the ambient temperature or by the constituent sub-components changing. Under certain circumstances, these changes may be so far-reaching that the number of oscillations, which are carried out by the oscillating circuit in a non-damped state, are so few because of these external circumstances that they are on a level with, or lower than, the number of oscillations which the oscillating circuit originally carried out in a damped state. In spite of attempts to compensate for this situation by means of various evaluation methods and with the location of the coils, the result has not been satisfactory. Among other things, the use of a large number of sensing coils may encroach upon the structural space in very small rotor devices. This applies in particular if, for reasons of space, the rotor has been designed such that it consists of part of the shaft of the device and, in that case, it is desired to have the coils located close to and directed axially to the end surface of the shaft. This surface may be very small in relation to the coils which are then to be accommodated there.
German patent specification DE 4137695 C2 describes a solution to the fact that the shape and the extent of the resonant oscillations are influenced not only by the damping, associated with the sensing of the rotation, but also by other external circumstances such as temperature variations and by the fact that the damping of the oscillations may vary, if the rotor has to be journalled with a radial play, which in this context is not negligible and which may have an unpredictable effect on the distance between the rotor surface and the coils. The described solution has been achieved by the use of a comparison component, which all the time compares the number of oscillations in consecutive resonant oscillations. In that case, it is only necessary to use two sensing coils, displaced 180°, or, in addition thereto, an additional third coil displaced 90°, if also the rotational direction is to be sensed. One problem remains, however, namely, if the coils, displaced 180°, pass the boundary between the electrically conductive and the electrically nonconductive surface parts of the rotor at the very moment when the sensing takes place. In that case, there is no presence of at least one oscillating circuit damped to a maximum extent and at least one oscillating circuit damped to a minimum extent. The evaluation may then become misleading. This applies particularly as it is often necessary to have a clearance in the bearing of the rotor and it is thus necessary to tolerate that the smallest distance between the respective coil and the electrically conductive surface part of the rotor varies during rotation of the rotor.